Purpose of Review

International ambitions for massive afforestation and restoration are high. To make these investments sustainable and resilient under future climate change, science is calling for a shift from planting monocultures to mixed forests. But what is the scientific basis for promoting diverse plantations, and what is the feasibility of their establishment and management? As the largest global network of tree diversity experiments, TreeDivNet is uniquely positioned to answer these pressing questions. Building on 428 peer-reviewed TreeDivNet studies, combined with the results of a questionnaire completed by managers of 32 TreeDivNet sites, we aimed to answer the following questions: (i) How and where have TreeDivNet experiments enabled the relationship between tree diversity and tree performance (including productivity, survival, and pathogen damage) to be studied, and what has been learned? (ii) What are the remaining key knowledge gaps in our understanding of the relationship between tree diversity and tree performance? and (iii) What practical insights can be gained from the TreeDivNet experiments for operational, real-world forest plantations?

Recent Findings

We developed a conceptual framework that identifies the variety of pathways through which target tree performance is related to local neighbourhood diversity and mapped the research efforts for each of those pathways. Experimental research on forest mixtures has focused primarily on direct tree diversity effects on productivity, with generally positive effects of species and functional diversity on productivity. Fewer studies focused on indirect effects mediated via biotic growing conditions (e.g. soil microbes and herbivores) and resource availability and uptake. Most studies examining light uptake found positive effects of species diversity. For pests and diseases, the evidence points mostly towards lower levels of infection for target trees when growing in mixed plantations. Tree diversity effects on the abiotic growing conditions (e.g. microclimate, soil properties) and resource-use efficiency have been less well studied to date. The majority of tree diversity experiments are situated in temperate forests, while (sub)tropical forests, and boreal forests in particular, remain underrepresented.

Summary

TreeDivNet provides evidence in favour of mixing tree species to increase tree productivity while identifying a variety of different processes that drive these diversity effects. The design, scale, age, and management of TreeDivNet experiments reflect their focus on fundamental research questions pertaining to tree diversity-ecosystem function relationships and this scientific focus complicates translation of findings into direct practical management guidelines. Future research could focus on (i) filling the knowledge gaps related to underlying processes of tree diversity effects to better design plantation schemes, (ii) identifying optimal species mixtures, and (iii) developing practical approaches to make experimental mixed plantings more management oriented.

中文翻译：

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通过物种混合提高树木性能：回顾四分之一世纪的 TreeDivNet 实验揭示了研究差距和实践见解

审查目的

国际上对大规模植树造林和恢复的雄心勃勃。为了使这些投资在未来气候变化下具有可持续性和弹性，科学界呼吁从种植单一作物转向混交林。但推广多样化种植园的科学依据是什么，建立和管理的可行性又是什么？作为全球最大的树木多样性实验网络，TreeDivNet 具有独特的优势来回答这些紧迫的问题。基于 428 项同行评审的 TreeDivNet 研究，并结合 32 个 TreeDivNet 站点的管理者完成的调查问卷结果，我们旨在回答以下问题：(i) TreeDivNet 实验如何以及在何处实现了树木多样性与树木性能之间的关系（包括生产力、存活率和病原体损害）需要研究，已经学到了什么？ (ii) 我们在理解树木多样性和树木性能之间的关系方面还存在哪些关键知识差距？ (iii) 从 TreeDivNet 实验中可以从现实世界的可操作森林种植园中获得哪些实际见解？

最近的发现

我们开发了一个概念框架，该框架确定了目标树木性能与当地社区多样性相关的各种途径，并绘制了每个途径的研究工作。森林混合物的实验研究主要集中在树木多样性对生产力的直接影响上，物种和功能多样性对生产力通常有积极的影响。很少有研究关注生物生长条件（例如土壤微生物和食草动物）以及资源可用性和吸收介导的间接影响。大多数研究光吸收的研究都发现了物种多样性的积极影响。对于病虫害，证据主要表明，在混合种植园中生长的目标树木的感染水平较低。迄今为止，树木多样性对非生物生长条件（例如小气候、土壤特性）和资源利用效率的影响还没有得到很好的研究。大多数树木多样性实验都发生在温带森林中，而（亚）热带森林，特别是北方森林的代表性仍然不足。

概括

TreeDivNet 提供了支持混合树种以提高树木生产力的证据，同时确定了驱动这些多样性效应的各种不同过程。 TreeDivNet 实验的设计、规模、年龄和管理反映了他们对与树木多样性与生态系统功能关系相关的基础研究问题的关注，而这种科学关注使研究结果转化为直接实用的管理指南变得复杂。未来的研究可以集中在（i）填补与树木多样性影响的基本过程相关的知识空白，以更好地设计种植方案，（ii）确定最佳物种混合物，以及（iii）开发实用方法，使实验性混合种植更加以管理为导向。